Low cost quick connectors for connecting tubing have been widely used in automotive and non-automotive applications. Typically, they are used to connect small diameter fluid lines with low pull-off requirements.
One example of such a quick connector is known as an interference type connector. This type of connector has expandable fingers mounted in a housing that create a mechanical interference on an upset portion or bead formed on the tube to be connected. The tube and upset are inserted into the connector housing past the expandable fingers. The fingers expand to allow the upset to pass and then retract behind the bead to retain the tube within the housing. The retention or pull-off force required to separate the tube from the housing is largely dependent upon the rigidity of the expandable fingers. Greater pull off force is required when there is greater resistance to finger expansion. On the other hand, the greater the rigidity of the expandable fingers, the more difficult it is to lock the tube within the housing. Further, fingers that are more rigid are more difficult to manually expand to remove the tube when removal is desired.
There are many applications which require very high pull-off forces, but low insertion forces. One such application is the connection of brake lines of a vehicle. No catastrophic failure such as bursting is permitted below forces of 500. Correspondingly, low insertion forces on the order of 15 pounds are required. Because of the need for high pull-off forces and low insertion forces, typical quick connectors are unacceptable for connecting brake lines. They are also unacceptable in other applications requiring high pull-off forces and low insertion forces.
For many high pressure applications, particularly with brake lines, threaded fittings are used. Cross threading can be a problem and the torque must be gaged. Otherwise, leaking may result which has to be repaired. Further, since the threads are mated, they must be twisted together. This increases assembly time and labor while simultaneously resulting in worker fatigue. To use threaded fittings there must be sufficient space for tools such as wrenches. It is difficult to use such tools in a blind location.
It is also difficult to separate threaded fittings. Many of the problems discussed with respect to their connection are present. Further, contaminant buildup between the fittings can make separation almost impossible. Thus, servicing is very difficult.